In piggyback trains, the cross-country travel is by rail and local delivery is by truck. This affects savings of energy and labor. However, the dead weight of the piggyback flat cars makes up a large portion of the total railway load. It has previously been proposed to provide novel highway trailers which can be supported upon a track without the use of a piggyback flat car. One such example is shown in U.S. Pat. No. 4,202,454 issued May 13, 1980. In this design, the highway trailer is provided with a plurality of wheel assemblies and is additionally provided with rail wheels which can be moved from a raised highway position to a lowered rail position. In addition to carrying the rail wheels and their associated axle, the highway trailer disclosed in this patent also has its weight increased by adding rail air reservoirs, rail air brake valving, rail brake shoes, rail brake beams, rail coupler actuating mechanisms, rail fixed pin coupler and rail tongue support systems, all of the various items being an integral part of the highway trailer. This additional mechanism to the highway trailer weight significantly increases its initial cost and also adds to the maintenance cost, and in effect, creates a vehicle that must be dedicated to rail service or for use near the rail thereby limiting its flexibility to be used as a general highway service vehicle. In addition, all of the above equipment which is added to the highway trailer significantly decreases its load carrying capability. In addition, the use of a single rail axle for each trailer restricts the rail use as it is customarily railroad practice and/or policy to limit freight loads to 30,000 pounds per wheel and thus the gross vehicle weight may not exceed 60,000 pounds.
Another prior art design is shown in U.S. Pat. No. 2,963,986 issued Dec. 13, 1960. In this design, a specially designed, non-standard railtruck is provided having a special adapter structure. A portion of the adapter structure can be telescoped into the rear end of a leading trailer and the fifth wheel king pin of a trailing trailer is also carried by the adapter on the railtruck. This design has failed to achieve commercial acceptance.